Ship Size and Ship Movement/Vibration questions

[Doc Tom]

We just returned from an 11 day Western Caribbean cruise on NCL's Epic (capacity ~4000).  During the first 36 hours in an Atlantic storm and during the last 24 hours (going fast to return to home port), my wife was quite uncomfortable (a tad nauseous and a lot anxious) due to the movement of and vibration in the ship.  Discussing this issue on the last night with a group of 10 "frequent cruisers" they advised us that movement & vibrations in mega-ships, like the Epic, are much worse because of the "huge sail area" and the "large tonnage" of these big ships. They recommended smaller ships such as Oceania's Sirena (capacity ~700), saying the smaller height & weight makes smaller ships much less likely to sway and vibrate. Does their logic make sense?  Appreciate your inputs. Thanks.

[LHT28]

Some ships  will have more motion depending on sea conditions also

 we have  had some rocking on Oceania  ships  both sizes

 

book cabins mid ship & lower deck to help  with the motion of the ocean

JMO

 

 

[Doc Tom]

4 minutes ago, LHT28 said:

Some ships  will have more motion depending on sea conditions also

 we have  had some rocking on Oceania  ships  both sizes

 

book cabins mid ship & lower deck to help  with the motion of the ocean

JMO

 

 

Thanks LHT. We are aware how cabin location affects rocking but were curious if there's a rule-of-thumb about mega-ships being less stable (the advice our "experienced" shipmates gave us).

[cruising cockroach]

Harmonic vibrations or was it from the diesel engines?

 

Usually 2 motions felt, pitch and roll, and a combination of the two (big ships unlikely to go up and down inbetween waves) and the higher you are up in the ships, the more likely you are to feel the movements, even spinning if pitch and roll are great enough.   Midship is better because you don't feel the up and down motion. 

[Doc Tom]

2 minutes ago, cruising cockroach said:

Harmonic vibrations or was it from the diesel engines?

 

Usually 2 motions felt, pitch and roll, and a combination of the two (big ships unlikely to go up and down inbetween waves) and the higher you are up in the ships, the more likely you are to feel the movements, even spinning if pitch and roll are great enough.   Midship is better because you don't feel the up and down motion. 

Hi CC, The movement seemed to be more roll than pitch (I think) and, as you say, there was little-to-no up & down motion.  I'm not knowledgeable enough to know the source of the vibrations although they did seem to be more random versus periodic. Thanks for your inputs.

[OzKiwiJJ]

We were on Majestic Princess (around 4000 pax) last November heading through a nasty little storm and barely felt any motion. It was quite surprising how little movement there was, yet we'd seen the size of the waves while having dinner in one of the speciality restaurants, and the winds were quite fierce as well. I've been in similar seas on Sun Princess and Celebrity Solstice, both smaller ships, and noticed far more motion. We booked cabins midships and on low to middle decks on those ships. We did have an aft cabin on Carnival Legend (also a smaller ship) in similar seas. Lots of movement, rattles, and vibration on that ship. 

Edited February 10, 2019 by OzKiwiJJ

[cruising cockroach]

As the first response stated, contemporary cruise ships have a shallow draft and a lot of surface area (high) above waterline so they will roll, not just due to wave action but catching winds too, unlike ocean liners that sit lower in the water.

 

Vibration could be just harmonics (annoying, I'll give you that), unless you were low in the ship and near the diesel engines.

[zqvol]

Physics tells you that the larger the surface area of the ship, the more it is affected by the wind. If the wind is coming from the side on the mega ships you can get a roll, though the stabilizers do a good job of damping it. On smaller ships, in the same weather you are going to bounce more.

 

The worst motion we have had has been on the smaller ships including Regent, Silversea and Sea Dream. It's just part of cruising.

 

The best solution is to take an anti-sea sick medicine.

[Doc Tom]

3 minutes ago, zqvol said:

Physics tells you that the larger the surface area of the ship, the more it is affected by the wind. If the wind is coming from the side on the mega ships you can get a roll, though the stabilizers do a good job of damping it. On smaller ships, in the same weather you are going to bounce more.

 

The worst motion we have had has been on the smaller ships including Regent, Silversea and Sea Dream. It's just part of cruising.

 

The best solution is to take an anti-sea sick medicine.

Thanks zqvol. That all makes sense.  Are the vibrations due to wind also?

[SantaFeFan]

7 minutes ago, Doc Tom said:

Are the vibrations due to wind also?

 

In part, yes. But much of it is due to the ship plowing through rough waves instead of smooth seas, which will create shaking on the hull that can be felt throughout the ship. Also, in rough weather the engines, propellers and rudders will have to work harder to move the ship forward and keep the ship on course through larger, more powerful waves which are constantly affecting ship handling. 

[Doc Tom]

6 minutes ago, SantaFeFan said:

 

In part, yes. But much of it is due to the ship plowing through rough waves instead of smooth seas, which will create shaking on the hull that can be felt throughout the ship. Also, in rough weather the engines, propellers and rudders will have to work harder to move the ship forward and keep the ship on course through larger, more powerful waves which are constantly affecting ship handling. 

SantaFeFan ---  You've described our experience exactly.  We felt a little rolling but a lot of shaking (perfect word: it was more of a shaking than a vibrating).  That makes a lot of sense now.  Thank you!

[Hlitner]

There are many factors at play, besides the size of the vessel.  Design plays a major role as many modern cruise ships are like huge wind vanes with a relatively shallow draft and lots of higher superstructure (for balconies).  As to vibration, that is really a complex issue.  It can come from mechanical vibration (which might be amidship) or from the props.  And even the props have different issues such as damage (which adds to vibration) or cavitation (a design issue).

 

But what gets ignored by many is that ships sail upon the sea and the sea can certainly move under various conditions.  When the sea moves, the ship moves.  Stabilizers and size can certainly dampen the movement, but wind and swell direction have a lot to say about movement.  And then there are the passengers.  DW and I have spend years (over 1000 days) on cruise ships and seldom mind ship movement.  But even on cruises that, to us, are hardly moving...we see folks complaining and turning green.  Part of the problem are the huge mega ships in the Caribbean that often are in smooth seas with no apparent movement.  Passengers feel really good about cruising and then get into a situation when the ship is moving because of 15+ foot swells (which will move any vessel).  For many seasoned cruisers that is a big nothing, but for others it is total misery.

 

Hank

[chengkp75]

Okay, lets address some of the factors involved here.

 

First, let's dispel some of the "cruise ship" versus "ocean liner" thing.  While there is only one "true" ocean liner according to CC members, the Queen Mary 2, she has a draft of 10.3 meters, while a slightly larger (though in Gross Tons, not necessarily in weight) Quantum of the Seas has a draft of 8.8 meters, or a difference of about 4 feet.

 

Next, with regards to "sail area" or the size of the ship above the water.  Unless the wind is gusting regularly, the force of the wind on the ship will be nearly constant, and therefore the ship will experience "heel", or a constant leaning away from the wind, not a "rolling", or repetitive lean from one side to the other.

 

Now, as to ship size.  Physics will tell you that it takes more force to start a large object moving than a small object, so a larger ship will not react to environmental forces as much as a smaller ship.  Next, ship stability is measured in meters of "GM" or the vertical distance between the ship's center of gravity and the center of buoyancy.  The greater this height difference, the "stiffer" the ship is, meaning it takes more force to start the ship rolling, and the more quickly the ship builds "righting moment" or the force to counteract the roll force and bring the ship upright again.  Cruise ships, due to their very high superstructures, tend to have large "GM" measurements, meaning they resist rolling, but when they do roll, they generate large righting moments which cause very violent and jerky rolling motion, which of course is uncomfortable to passengers.  This is the purpose of stabilizers, to "slow the roll" to comfortable levels.  Stabilizers, of any type used on ships, require some roll motion to react to, and do not stop or reduce roll, they only prolong the time of the roll, smoothing it out.

 

And finally, vibrations.  The vast majority of vibration felt onboard a ship will be from the propellers.  Even in flat calm seas, there will be some vibration from cavitation as the propeller turns at 125+- rpms.  Then there will be "blade passage" vibration, where each time a propeller blade passes the top of its rotation, the blade compresses the water between itself and the ship's hull, and this pressure is passed to the ship's hull and sets up a vibration.  If the frequency of these blades passing the hull is at the same frequency as the hull's "natural frequency", then a harmonic vibration of larger amplitude will be set up (think of a guitar string causing the entire guitar box to amplify it's vibration).   When you have more than one propeller, you compound the possibility of harmonic vibrations both because each propeller can feed to the hull's natural frequency, but also the two propellers will interact with each other's pressure waves.

 

Add to this the relatively new phenomenon of "azipod shimmy", where the actual propellers are steered port to starboard to steer the ship, and this directs the water flow from under the hull into the propellers from varying directions as the propellers track back and forth in a sea way to keep course, and you get a "side to side" vibration, particularly near the after end of the ship.

 

And finally, there is weather.  As the ship pitches (longitudinal bow down, stern up, and vice versa), the propellers will rise and fall in the water, getting closer to the surface when the stern lifts, and deeper in the water when the stern goes down.  As the propeller gets shallower, it has less resistance in the water, and will tend to speed up, and lose propulsive force, until the speed governor catches it.  When it gets deep, the resistance increases, and the propeller works harder to move the ship, and slows slightly.  This periodic speed up, slow down, and less force, more force, creates a vibration of its own.

 

All of these vibrations will be transmitted to the ship's hull structure, along with vibrations from waves striking the ship, and the hull is essentially a large box, so like a musical instrument, it will transmit those vibrations throughout the hull, and possibly amplify them if the frequency is right.  Even among "sister ships" that to all appearances are identical, there will be differences in the harmonic frequency of the ship, at different locations around the ship, so two "sisters", in the exact same environmental conditions (sailing side by side), can experience different vibration signatures.

 

So, to end this, while smaller ships will have less powerful propulsion systems, and this may decrease some vibrations, its not a given, and the smaller ship will be more susceptible to environmental forces moving it, "swaying", than a larger ship.

[Doc Tom]

2 hours ago, chengkp75 said:

Okay, lets address some of the factors involved here.

 

First, let's dispel some of the "cruise ship" versus "ocean liner" thing.  While there is only one "true" ocean liner according to CC members, the Queen Mary 2, she has a draft of 10.3 meters, while a slightly larger (though in Gross Tons, not necessarily in weight) Quantum of the Seas has a draft of 8.8 meters, or a difference of about 4 feet.

 

Next, with regards to "sail area" or the size of the ship above the water.  Unless the wind is gusting regularly, the force of the wind on the ship will be nearly constant, and therefore the ship will experience "heel", or a constant leaning away from the wind, not a "rolling", or repetitive lean from one side to the other.

 

Now, as to ship size.  Physics will tell you that it takes more force to start a large object moving than a small object, so a larger ship will not react to environmental forces as much as a smaller ship.  Next, ship stability is measured in meters of "GM" or the vertical distance between the ship's center of gravity and the center of buoyancy.  The greater this height difference, the "stiffer" the ship is, meaning it takes more force to start the ship rolling, and the more quickly the ship builds "righting moment" or the force to counteract the roll force and bring the ship upright again.  Cruise ships, due to their very high superstructures, tend to have large "GM" measurements, meaning they resist rolling, but when they do roll, they generate large righting moments which cause very violent and jerky rolling motion, which of course is uncomfortable to passengers.  This is the purpose of stabilizers, to "slow the roll" to comfortable levels.  Stabilizers, of any type used on ships, require some roll motion to react to, and do not stop or reduce roll, they only prolong the time of the roll, smoothing it out.

 

And finally, vibrations.  The vast majority of vibration felt onboard a ship will be from the propellers.  Even in flat calm seas, there will be some vibration from cavitation as the propeller turns at 125+- rpms.  Then there will be "blade passage" vibration, where each time a propeller blade passes the top of its rotation, the blade compresses the water between itself and the ship's hull, and this pressure is passed to the ship's hull and sets up a vibration.  If the frequency of these blades passing the hull is at the same frequency as the hull's "natural frequency", then a harmonic vibration of larger amplitude will be set up (think of a guitar string causing the entire guitar box to amplify it's vibration).   When you have more than one propeller, you compound the possibility of harmonic vibrations both because each propeller can feed to the hull's natural frequency, but also the two propellers will interact with each other's pressure waves.

 

Add to this the relatively new phenomenon of "azipod shimmy", where the actual propellers are steered port to starboard to steer the ship, and this directs the water flow from under the hull into the propellers from varying directions as the propellers track back and forth in a sea way to keep course, and you get a "side to side" vibration, particularly near the after end of the ship.

 

And finally, there is weather.  As the ship pitches (longitudinal bow down, stern up, and vice versa), the propellers will rise and fall in the water, getting closer to the surface when the stern lifts, and deeper in the water when the stern goes down.  As the propeller gets shallower, it has less resistance in the water, and will tend to speed up, and lose propulsive force, until the speed governor catches it.  When it gets deep, the resistance increases, and the propeller works harder to move the ship, and slows slightly.  This periodic speed up, slow down, and less force, more force, creates a vibration of its own.

 

All of these vibrations will be transmitted to the ship's hull structure, along with vibrations from waves striking the ship, and the hull is essentially a large box, so like a musical instrument, it will transmit those vibrations throughout the hull, and possibly amplify them if the frequency is right.  Even among "sister ships" that to all appearances are identical, there will be differences in the harmonic frequency of the ship, at different locations around the ship, so two "sisters", in the exact same environmental conditions (sailing side by side), can experience different vibration signatures.

 

So, to end this, while smaller ships will have less powerful propulsion systems, and this may decrease some vibrations, its not a given, and the smaller ship will be more susceptible to environmental forces moving it, "swaying", than a larger ship.

Wow, chengkp75 -- thanks for the comprehensive answer.

[cruising cockroach]

4 hours ago, chengkp75 said:

Okay, lets address some of the factors involved here.

 

First, let's dispel some of the "cruise ship" versus "ocean liner" thing.  While there is only one "true" ocean liner according to CC members, the Queen Mary 2, she has a draft of 10.3 meters, while a slightly larger (though in Gross Tons, not necessarily in weight) Quantum of the Seas has a draft of 8.8 meters, or a difference of about 4 feet.

 

Would it not be fair to observe that newer/contemporary cruise ships have a much higher ratio of height above water line than below, compared to older cruise ships that may have been designed as liners?   Wasn't there a liner that was used as a cruise ship until it was determined ts draft was too deep to get into a lot of ports?

 

Of course the CGs and stability are the same but you will feel a lot more movement due to distance from CG than you would in a smaller, lower ship?

 

 

 

2 hours ago, Doc Tom said:

 

 

[chengkp75]

1 hour ago, cruising cockroach said:

 

Would it not be fair to observe that newer/contemporary cruise ships have a much higher ratio of height above water line than below, compared to older cruise ships that may have been designed as liners?   Wasn't there a liner that was used as a cruise ship until it was determined ts draft was too deep to get into a lot of ports?

 

Of course the CGs and stability are the same but you will feel a lot more movement due to distance from CG than you would in a smaller, lower ship?

 

 

 

 

Yes, they may be taller than older ships, but what determines the depth underwater is the weight of the ship, and its length and beam, not its vertical size.  As an example, the SS United States has a displacement (the actual weight of the ship) of 47,000 tons, and a draft of 9.8 meters, and this is considered to be one of the finest liners around.  The Oasis of the Seas has a displacement about 2 times as much, about 100,000 tons, and has a similar draft of 9.3 meters, so twice as much weight is spread along a longer hull and a wider hull.  Given the similar drafts, the greater beam (47 meters v 30 meters, or 150%) means the center of buoyancy for Oasis is lower in the ship than the US, which improves stability.  So, a ship with more height, and a proportionally smaller draft can have more stability.

 

Now, if you are talking about actual linear motion when rolling, then yes, the farther you are from the center of gravity then you will move a linear distance farther from the upright location, as you are moving on an arc, so the greater the radius the greater the arc length.  However, as you say, this is based on distance from the center of gravity, and taller ships have a center of gravity higher than shorter ships.  So, a shorter ship may have a center of gravity around deck 2, while a taller ship may have it around deck 4, a lot depends on construction method and material, and what is in those decks.  And the ship's draft has nothing to do with where the center of gravity is.  And no, the center of gravity nor the stability will be the same between a taller ship and a shorter ship with the same or nearly the same drafts.  Draft determines the center of buoyancy, not gravity.

[puppycanducruise]

Thanks Chief.

Great info, as always.